Real-ear measurement of hearing threshold and loudness.

Abstract

For at least the past sixty years, the predominant type of electronic circuit present in hearing aids has been a single-channel amplifier designed to be linear over most of its operating range with its maximum output limited via peak clipping (Berger, 1984). Although this is presently still the case (Hawkins and Naidoo, 1993), it is apparent that hearing aids designed to be nonlinear over most of their operating range are increasingly encountered and are now offered by a wide range of manufacturers (Berkey et al, 1992; HIA, 1995). Moreover, many of these contemporary devices are two- or three-channel instruments with programmable control of the nonlinear characteristics available for each channel. Research into the physiological and perceptual nature of the sensorineural hearing loss experienced by the vast majority of hearing-aid wearers (see review by Van Tasell, 1993) and advancements in engineering (see Killion, 1993) have largely led to this trend toward nonlinear hearing aids. Briefly, it has been established that the normal inner ear provides a compression of the input sound intensity that appears to be dependent on the presence of normally functioning outer hair cells. Listeners with sensorineural hearing loss attributed to underlying cochlear pathology typically lack normally functioning outer hair cells and the corresponding compression of input sound intensity. Nonlinear hearing aids have been designed to restore this normal compression of sound intensity prior to delivering the amplified sound to the inner ear of the listener with sensorineural hearing loss. The ways in which hearing aids are fit and evaluated have also undergone considerable evolution over the past sixty years. This evolution, primarily concerned with the fitting of linear hearing aids, has been reviewed recently elsewhere (Humes, 1996). The most commonly accepted approach to fitting linear hearing aids involves the use of some prescriptive formula, based either on thresholds or suprathreshold loudness measurements [see Humes (1991), McCandless (1994), and Humes and Halling (1994) for reviews], to generate target real-ear gain. Once targets are generated, the hearing aid is fit to the patient and adjusted to achieve a match to the targeted gain values for a moderate level input signal. For contemporary nonlinear hearing aids, unlike their linear predecessors, the gain at low and high input levels is not designed to be the same as that at the moderate input levels used to generate and confirm the targeted values. For nonlinear devices, the gain is generally less as the input increases over the range from 50 to 100 dB SPL, although the manner in which it decreases with increases in input level varies with the particular circuit incorporated into the hearing aid. Moreover, the desired level-dependent gain characteristics may vary across channels for multi-channel hearing aids.